The present invention relates to a toroidal-type continuously variable transmission and, in particular, to an improvement in a toroidal-type continuously variable transmission for use in vehicles such as automobiles.
As a conventional toroidal-type continuously variable transmission, there is known such a toroidal-type continuously variable transmission which is shown in FIG. 1.
In FIG. 1, reference characters 1 and 2 respectively designate an input disk and an output disk which are coaxially disposed so as to be opposed to each other within a housing (not shown). An input shaft 3 is penetrated through the axial portion of the toroidal-type continuously variable transmission in which the input and output disks 1 and 2 are disposed. In one end of the input shaft 3, there is disposed a loading cam 4. And, the loading cam 4 transmits the motive power (rotational force) of the input shaft 3 to the input disk 1 through a cam roller 5.
The input and output disks 1 and 2 have substantially the same shape and disposed symmetric; and, the mutually opposing surfaces of the input and output disks 1 and 2 are formed in toroidal surfaces which cooperate together in forming a substantially semicircular shape when they are viewed from the axial-direction cross sections thereof. And, in a toroidal cavity which is defined by the toroidal surfaces of the input and output disks 1 and 2, there are disposed a pair of power roller bearings 6 and 7 for power transmission in such a manner that they are in contact with the input and output disks 1 and 2.
Here, the power roller bearing 6 comprises a power roller 6a (which corresponds to an inner race forming part of the power roller bearing 6) which is disposed so as to roll on the toroidal surfaces of the input and output disks 1 and 2, an outer race 6b and a plurality of rolling bodies (steel balls) 6c. On the other hand, the power roller bearing 7 comprises a power roller 7a (which corresponds to an inner race forming part of the power roller bearing 7) which is disposed so as to roll on the toroidal surfaces of the input and output disks 1 and 2, an outer race 7b and a plurality of rolling bodies (steel balls) 7c. 
That is, the power roller 6a serves also as an inner race which is a component of the power roller bearing 6, while the power roller 7a serves also as an inner race which is a component of the power roller bearing 7.
In this structure, the power roller 6a is pivotally mounted on a trunnion 10 through a pivot shaft 8, the outer race 6b and the plurality of rolling bodies 6c, and it is also supported in such a manner that it can be incliningly rotated about a pivot shaft O which provides the center of the toroidal surfaces of the input and output disks 1 and 2. On the other hand, the power roller 7a is pivotally mounted on a trunnion 11 through a pivot shaft 9, the outer race 7b and the plurality of rolling bodies 7c, and it is also supported in such a manner that it can be incliningly rotated about a pivot shaft O which provides the center of the toroidal surfaces of the input and output disks 1 and 2.
And, to the contact surfaces between the input and output disks 1, 2 and the power rollers 6a, 7a, there is supplied lubricating oil which has large viscous friction resistance; and thus the motive power input to the input disk 1 can be transmitted to the output disk 2 through films of the lubricating oil and power rollers 6a and 7a. 
By the way, the input and output disks 1 and 2 are formed to be independent of the input shaft 3 (that is, they are not directly affected by the motive power input to the input shaft 3, namely, a rotary shaft) due to interposition of needles 12. In the output disk 2, there is disposed an output shaft 14 which is arranged in parallel to the input shaft 3 and is also rotatably supported on a housing (not shown) through angular bearings 13.
In the toroidal-type continuously variable transmission 20, the motive power of the input shaft 3 is transmitted to the loading cam 4. And, when the loading cam 4 is rotated due to the transmission of the motive power, the rotational power of the loading cam 4 is transmitted to the input disk 1 through the cam roller 5, thereby rotating the input disk 1. Further, the rotational power of the input disk 1 is transmitted to the output disk 2 through the power rollers 6a and 7a. In response to this, the output shaft 14 is rotated integrally with the output disk 2. To change the speed of a vehicle, the trunnions 10 and 11 may be moved by a slight distance in the direction of the pivot shaft 0.
That is, in case where the trunnions 10 and 11 are moved in the axial direction, the intersection between the rotating shaft of the power rollers 6a, 7a and the axis of the input and output disks 2 is caused to shift by a slight amount. This loses equilibrium between the rotational peripheral speed of the power rollers 6a, 7a and the rotational peripheral speed of the input disk 1 and also, due to a component of the rotational drive power of the input disk 1, the power rollers 6a and 7a are incliningly rotated around the pivot shaft O.
Thus, the power rollers 6a and 7a rotate incliningly on the curved surfaces of the input and out disks 2, whereby the speed ratio is changed, that is, the vehicle can be decelerated or accelerated. As a toroidal-type continuously variable transmission which includes the above-mentioned structure, conventionally, there is known a toroidal-type continuously variable transmission which is disclosed in JP-B-2-49411. Also, as the input and output disks as well as power roller bearings of the above-described type, conventionally, there are known disks and power roller bearings which, as set forth in [NASA Technical note NASA ATN-8362], use AISI52100 (which corresponds to JISSUJ2, high-carbon chromium bearing steel).
However, in the above-mentioned conventional toroidal-type continuously variable transmission, when driven, between the input disk 1 and its associated power roller bearing as well as between the output disk 2 and its associated power roller bearing, there are applied a high contact pressure as well as a high bending stress. In case where such contact pressure and bending stress are applied to the disks or power rollers, in the disks and power rollers, with non-metallic inclusions existing in the interior portions thereof as crack start points, cracks can be produced and developed, with the result that the disks and power rollers can be damaged or broken.
Thus, the present invention aims at eliminating the drawbacks found in the above-mentioned conventional toroidal-type continuously variable transmission.
Accordingly, it is an object of the invention to provide a toroidal-type continuously variable transmission, in which it is possible to enhance the fatigue or breakage lives of the power roller bearings and in turn greatly extend the life of the toroidal-type continuously variable transmission.
In attaining the object, according to the invention, there is provided a toroidal-type continuously variable transmission, comprising:
an input disk and an output disk disposed coaxially so as to be opposed to each other; and,
a pair of power roller bearings, each power roller bearing including an inner race, an outer race and a plurality of rolling elements disposed so as to roll on the toroidal surfaces of the input and output disks,
wherein at least one of the input disk, the output disk, the inner race and the outer race is made of medium-carbon steel containing carbon in an amount ranging from 0.3 wt % to 0.5 wt %.
In the above-mentioned toroidal-type continuously variable transmission according to the present invention, it is advantageous that the at least one of the input disk, the output disk, the inner race and the outer race has a surface layer which is carburized or carbonitrided.
In addition, in the above-mentioned toroidal-type continuously variable transmission according to the present invention, it is preferable that the at least one of the input disk, the output disk, the inner race and the outer race has a surface layer which is carburized or carbonitrided and are further hardened and tempered in such a manner that its resultant core portion hardness can be HRC 35 or higher.
Further, in the above-mentioned toroidal-type continuously variable transmission according to the present invention,: it is advantageous that assumed maximum non-metallic inclusion diameters of the medium-carbon steel in an area of 3000000 mm2 is at 40 xcexcm or less, wherein the assumed maximum inclusion diameters are determined by an extreme statistical method which is carried out by checking 30 pieces of its samples while using a microscope, measuring the maximum inclusions in an area of 100 mm2 in each samples, and finding the assumed inclusion diameters in an area of 3000000 mm2through the measurement of a total area of 3000 mm2.
Furthermore, in the toroidal-type continuously variable transmission according to the present invention, it is preferable that the medium-carbon steel is selected from the group consisting of SCM 415, SCM 420, SCM 430, SCM 435, SCM 440, SCM 445, and a modified version of SCM 445 with its carbon content increased up to 0.6%.